the way I store private keys
1. I use a web wallet (Myetherwallet)
2. Bookmark the web wallet site to avoid phishing attacks
3. copy on white paper, PC, MOBILE and flashdisk
4. I will not tell anyone and only I personally know about this.
If you are using web wallet then your private keys is not safe, if you are thinking its safe you are wrong, unless you don't give full details, web wallets are wallets that still requires for login to access the wallet even if they use private keys,this is why its risky cos anybody can get access to your login details, why not use standalone wallets? the app version on myetherwallet is even better